Low leakage inductance transformer for conductive parallel plate ballanced transmission lines



June 1 7, T969 3,451,013 T'IVE s of 2 L. P. LEARNEY ETAL LOW LEAKAGE INDUCTANCE TRANSFORMER FOR CONDUC PARALLEL PLATE BALANCED TRANSMISSION LINE Filed April 3, 1967 Sheet "K I I 3 I 3,451,013 UGTI TRANSMISSION LINES June 17, 1969 L. P. LEARNEY ETAL LOW LEAKAGE INDUCTANCE TRANSFORMER FOR COND PLATE BALANCED Sheet PARALLEL Filed April 3, 1967 United States Patent US. Cl. 333-25 12 Claims ABSTRACT OF THE DISCLOSURE The invention is a wide frequency band transformer, e.g. 100-30,000 kc./s., for conductive plate type balanced transmission lines, having low leakage inductance. The transformer has a toroidal core positioned in aligned holes in the three transmission line plates. It has windings comprising two sets of straps, angularly spaced apart around the core axis, extending axially around the core, all straps having one end connected to the middle plate of the transmission line and alternate straps having their opposite end connected to different ones of the outer plates. It has multiple turn windings passing axially around the core and through holes in all the plates.

This invention is an improvement of the invention described in our copending British patent application No. 24,000 filed May 27, 1966, and relates to electric transformers for transmitting a band of frequencies, for use in conjunction with transmission lines having elements in the form of conductive plates.

In radio apparatus in which currents in a wide frequency band have to be transmitted, for example from 100 to 30,000 kilocycles per second, it is customary to use transmission lines having the form of conductive plates, in circuits of the apparatus, where the characteristic impedance is required to have a value of the order of a few ohms.

Hitherto, difficulty 'has been experienced in designing transformers for feeding current into or out of such transmission lines, to have high efficiency over the whole frequency band, because of excessive leakage inductance in the transformers and the presence of unwanted inductance in leads connecting the transformers to the lines. For example, a connecting lead having an inductance of only 1 microhenry, has a reactance of 2 ohms at 30,000 kilocycles per second.

It is an object of the present invention to provide an improved transformer for feeding current into or out of a transmission line of the form having conductive plates separated by a dielectric, in which the ratio leakage inductance/ shunt inductance is reduced and in which the winding feeding current into or out of the transmission line is of a form such that its ends can be directly connected to the plates of the line.

It is a further object of the present invention to provide an improved transformer for feeding current into or out of a balanced transmission line of the form having conductive plates separated by a dielectric, in which increased magnetic coupling is provided between a first winding of the transformer and a second winding of the transformer adapted to provide balanced input or output connections for the line.

Accordingly, the present invention provides an electric transformer for transmitting a band of frequencies, for use in conjunction with a balanced transmission line of the form having at least two outer plates and an intermediate plate of conductive material each of the outer plates being separated from the intermediate plate by a dielectric, comprising a closed core of magnetic material and at least two windings, the core being supported within a first winding having two halves, the turns of which are uniformly distributed along the length of the core in a manner such that the turns of one half of the winding are positioned between the turns of the other half of the winding, one end of the first winding being connected to one otuer plate of the transmission line, along the edge of a hole in the plate, the other end of the first winding being connected to the other outer plate of the transmission line, along the edge of a hole in the plate, the junction between the two halves of the first winding being connected to the intermediate plate of the transmission line, along the edge of a hole in the intermediate plate, a plurality of multi-turn coils connected to provide at least one other winding, the turns of the said coils being wound over the said first winding and through a hole or a series of holes provided in the transmission line around the first winding.

In order that the invention may be more readily carried into effect, an embodiment thereof will now be described in detail, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is an isometric view of a radio frequency output transformer, according to the invention, assembled into a balanced transmission line of plate form, in which the primary winding only is shown and parts of the transmission line and transformer are shown cut away for the sake of clarity and for convenience of explanation;

FIG. 2 is an isometric view of the primary winding of the transformer of FIG. 1 before assembly; and

FIG. 3 is an isometric view of the transformer of FIG. 1, in which both primary and secondary windings are shown.

Referring to FIG. 1, a balanced type transmission line, shown generally by the reference number 10, is formed by assembling, in a sandwich, three plates 11, 12 and 13 of highly conductive metal such as copper. The plate 12, which is located between the plates 11 and 13 is formed from two metal sheets 12 and 12" of the same area, with two of their surfaces in contact. The sheets are riveted or soldered together to provide good electrical connection between them. The plate 12 is formed in this manner for areason which will be described in detail later in this specification.

A dielectric of insulating material, which is not shown in the drawing, of low loss material such as polytetrafiuoroethylene is inserted between the plates 11 and 12 and between the plates 12 and 13.

For the sake of clarity, the distances between the plates 11 and 13 and the plate 12, have been exaggerated in the figure.

As is generally known to those experienced in the art, the characteristic impedance of a plate type transmission line is determined by the width of the metal plates and by the thickness and dielectirc constant of the insulating means.

At a convenient location, near to an extremity of the line, in this example, holes are cut in the plates 11, 12 and 13 of a diameter to accommodate a core 15 and a primary winding 16 of a transformer, indicated generally by the reference number 14. The core 15, of toroidal form, is partially enclosed within the winding 16. The winding 16 is formed from sheet metal in a manner to be described in detail later in this specification. The core 15, comprises one or more annular rings of magnetic material, for example of manganese zinc ferrite, shrouded in a thin covering of insulating material such as nylon. The size and type of the core material is chosen according to the power to be transmitted by the transformer, the frequency range and the maximum flux density permissible in order to avoid distortion of the waveform.

The winding 16 comprises eight turns 31 to 38, of which seven turns 31 to 37, are shown in the drawing. The diameter of the hole in the plate 11 exceeds the diameter of the hole in the plate 12 by approximately 0.125 inch so as to allow each of the four turns 31 to 34, to be connected at one end to the plate 12, to pass through the gap between the core 15 and the edge of the hole in the plate 11, to be wound in the clockwise direction around the perimeter of the section of the core and to be connected at the other end to the plate 13 near to the edge of the hole in the plate. Similarly, the diameter of the plate 13 exceeds the diameter of the hole in the plate 12 by approximately 0.125 inch, so as to allow each of the four turns to 38, to be connected at one end to the plate 12, to be wound in the counterclockwise direction around the perimeter of the section of the core and to be connected at the other end to the plate 11 near to the edge of the hole in the plate. Thus, the primary winding 16 of the transformer is a centretapped winding, each half of which comprises four turns connected in paraliel, the turns of the two halves of the winding being uniformly distributed around the core so that the turns of one half of the winding are positioned between the turns of the other half of the winding. The two ends of the winding are connected to the plates 11 and 13 and the common centre-tap to the plate 12, hence, the load on the line is symmetrical and the electrical balance of the line is not disturbed.

Referring to FIG. 2, the primary winding of the transformer, is formed from a rectangular sheet of copper having a thickness of 0.012 inch. The sheet is slotted along one longer edge to provide a structure having a strip-like body 39 and eight equi-spaced limbs 31 to 38, at right angles thereto. The turns of the primary winding are formed from the eight limbs, hence, the turns and the limbs are indicated by the same reference numbers 31 to 38, in the drawing.

Bends, are made in the four limbs 31 to 34 in the same direction and, in the four limbs 35 to 38, in the opposite direction along the junction between the body and the limbs, sothat the surfaces of the body and the surfaces of the limbs are at right angles and the surfaces of the limbs are in the same plane.

The body 39 is of a length and width indicated in the drawing by dimension lines 30 and 30 respectively and reference letters A and B respectively. The length A of the body is approximately equal to the circumference of the outer surface of the core 15. In this example, the length A is 5 inches and the width B is approximately 0.3 inch. In a simple form of the primary winding 16, the limbs 31 to 38 are of uniform width, the width of each of the limbs being somewhat less than one eighth of the length of the circumference of the inner surface of the core 15, so that gaps are provided between the turns and shorting between adjacent turns is avoided. In this example, the limbs are shaped to have greater width near to their extremities and near to the junction of the limbs with the body 39, so as to provide greater strength and increase coupling between the turns and the core.

A further right-angle bend is made in each of the limbs along a line approximately 0.3 inch from the extremity of the limb, in the same direction as that of the first bend. The length of each limb between the right-angle bends is approximately equal to the length of the perimeter of the cross section of the core.

Eight saw cuts 40 to 47 are provided in the body 39 in line with the centres of the limbs 31 to 38, so that the body 39 can be shaped to take the form of a polygon and lie around the edge of the hole in the plate 12 in which the core and winding are accommodated.

Eight holes 48 to 55, of elliptical shape, are also provided in the body 39. The holes are positioned in the body 39 so that their major axes lie On the centre line CC of the body and their centres lie on the centre lines of the limbs 31 to 38. A further eight holes 56 to 63, of the same shape and size as the holes 48 to 55, are provided near to the extremities of the limbs. The holes are positioned so that their major axes lie on a line approximately 0.15 inch from the end of each of the limbs and their centres lie on the centre lines of the limbs.

Referring again to FIG. 1, as mentioned earlier in this specification, a hole of a diameter to accommodate the core 15 and the winding 16, is provided in the inner plate 12 of the transmission line 10. In order that the shaped body 39 of the winding 16 may be connected to the inner plate without producing non-uniform spacing between the inner plate 12 and the outer plate 11, the centre plate 12 is formed from two metal sheets 12' and 12" joined together, each sheet having the thickness half that of the plate 11 or the plate 13. A hole is provided in the metal sheet 12' having a diameter approximately 0.5 inch greater than that of the hole in the metal sheet 12". The body 39, after shaping, is accommodated in the recess which is formed when the metal sheets are joined together. The body is soldered to the metal sheet 12" so as to provide a good electrical connection between the plate 12 and the centre-tap of the winding. Each of the limbs 31 to 34 is wrapped around the perimeter of the core cross-section and the ends of the turns so formed are joined by means of solder near to the edge of the hole in the outer plate 13. Similarly, each of the limbs 35 to 38 is wrapped around the perimeter of the core cross-section in a direction opposite to that of the turns 31 to 34, and the ends of the turns so formed are joinedi by means of solder near to the edge of the hole in the outer plate 11 (FIG. 1). Thus, the limbs 31 to 38 provide a centre-tapped winding, the turns of which are wound close to the core and which extend along substantially the whole length of the core, so that tight magnetic coupling between the winding 16 and the core is achieved. The number of turns need not be restricted to a total of eight turns. However, as will be evident from the results of tests given later in this specification, eight turns have been found suflicient to provide adequate coupling between the halves of the winding.

In FIG. 1, eight evenly spaced and aligned holes of elliptical shape are provided in the plates 11, 12 and 13 and in the dielectric, around the outer edge of the primary winding, to enable turns of insulated covered wire forming separate sections of a secondary winding to be wound over the winding 16. In the drawing, four holes 17, 18, 20 and 21 only are shown. The eight evenly spaced holes are punched in each of the plates 11, 12 and 13 and in each of the insulating sheets separating them before the line is assembled. As already described, holes of similar size and shape are provided near to the extremities of the limbs 31 to 38, and along the length of the body 39. These holes are aligned with the holes in the plates 11, 12 and 13 and in the insulating sheets separating them, when the core and the winding 16 is assembled into the transmission line. To prevent chafing of the insulation, each hole is lined with insulating material, not shown in the drawing.

Referring to FIG. 3, the transformer 14 is provided with a secondary winding of eight sections, indicated by reference numbers 70 to 77. Each section has five turns of enamel-led copper wire, wound over the primary winding and passed through one of the eight slots. The primary winding has previously been covered with a protective layer 78 of the insulating material, for example, po'lytetrafluoroethylene sheet. The protective layer is provided using two parts, each part having the form of an annulus with radial cuts around the inner and outer circumferences of the annulus to allow the material to be folded over the primary winding 16. Five holes only, 17, 18, 19, 20 and 21 associated with the coils, 70, 77, 76, 75 and 74 respectively, are shown in the drawing. Three only of four connections between the ends of the turns 35 to 38, and the plate 11 are visible in the draw ing and are indicated by the reference numbers 37, 38 and 35.

In order to keep leakage inductance to a minimum, the secondary sections are distributed around the core. The eight windings, each of five turns of wire, are connected electrically in parallel. For this purpose, .a ring 79, of 16 S.W.G. tinned copper wire is provided, to which the start of each winding is connected.

A similar ring, not shown in the drawing, is provided to which the finish of each winding is connected. Connections to external apparatus are made at convenient positions on the two rings. One such connection is shown in the drawing and is indicated by the reference number 80.

The transformer 14 may be used either for transferring radio frequency energy from an external source into the transmission line or for transferring radio frequency energy from the transmission line to an external load.

The choice of a suitable ferrite core 15 for use with the radio frequency transformer 14, shown in FIGURE 1, is made by reference to technical information issued by the manufacturers of such cores. This information includes physical properties of the core such as permeability and power loss.

The volume of the core and the number of turns necessary to obtain a desired value of inductance is determined using well-known formulae. The inductance of the primary winding is chosen to have a value such that the reactance of the winding connected to the line, at the lowest frequency, presents a negligible shunt on the characteristic impedance of the transmission line.

In an example of a transformer constructed according to the invention, the transformer functions as a radio frequency output transformer, providing optimum transfer of power from a transmission line having a characteristic impedance of 12.5 ohms into an external resistive load of 50 ohms.

A core of manganese zinc ferrite is used, having an outside diameter of approximately 38 millimetres and crosssectional area of approximately 40 square millimetres, the transformer is accommodate in a plate type transmission line having a length of approximately 9 inches and a width of approximately 2 inches .and is capable of delivering powers of up to approximately 100 watts into a load of 50 ohms over a range of the frequencies from 1,500 to 30,000 kilocycles per second.

In this arrangement, a primary inductance of 4.8 microhenries and a secondary inductance of 29 microhenries is obtained. With the whole primary shorted, the leakage inductance is less than 0.25 rmicrohenry and with half of the primary shorted, the leakage inductance is less than 0.5 microhenry.

Further windings may be provided for feeding radio frequency potentials of low amplitude to an external measuring device and for feeding a feedback potential to input electrodes of amplifying means connected to the transmission line 10.

In a further alternative arrangement, the core is of rectangular cross-section and of square form, and is almost totally enclosed within a multi-turn winding of corresponding shape, the secondary sections being wound over the multi-turn primary along the straight portions of the turns.

What we claim is:

1. An electric transformer for transmitting a band of frequencies, for use in conjunction with a balanced transmission line of the form having at least two outer plates and an intermediate plate of conductive material each of the outer plates being separated from the intermediate plate by a dielectric, comprising a closed core of magnetic material and at least two windings, characterised in that the plates have first aperture portions, dimensioned to receive said magnetic core, and at least one further aperture portion, the core being supported within a first winding having two halves, the turns of which are uniformly distributed along the length of the core in a manner such that the turns of one-half of the winding are positioned between the turns of the other half of the winding are positioned between the turns of the other half of the winding, one end of the first winding being connected to one outer plate of the transmission line, along the first aperture defining edge of the plate, the other end of the first winding being connected to the other outer plate of the transmission line, along the first aperture defining edge of that plate, the junction between the two halves of the first winding being connected to the intermediate plate of the transmission line, along the first aperture defining edge of the intermediate plate, .a plurality of multi-turn coils connected to provide at least one other winding, the turns of the said coils being wound over the said first winding and through a further said aperture of the plates.

2. An electric transformer as claimed in claim 1, characterised in that said first winding is formed from a single rectangular metal sheet.

3. An electric transformer as claimed in claim 2, characterised in that the said metal sheet is slotted to provide a plurality of limbs which are attached to a marginal portion of the metal sheet at one end and are free at the other end, said limbs being bent, initially, into a plane at right angles to the plane of said marginal portion, alternate limbs extending in opposite directions in said plane.

4. An electric transformer as claimed in claim 3, characterised in that said marginal portion is slotted to facilitate bending of said portion into fiat annular shape.

5. An electric transformer as claimed in claim 4, characterised in that the extremities of said limbs are bent at right angles to the adjacent limb part.

6. An electric transformer as claimed in claim 5, characterised in that the said limb extremities and the said marginal portion are apertured.

7. An electric transformer .as claimed in claim 3, characterised in that said limbs are of reduced with intermediately of the free extremity and the attached end of each thereof.

8. An electric transformer as claimed in claim 3, characterised in that the said marginal portion is connected to the intermediate plate of the transmission line and the free extremities of alternate limbs are connected to opposite outer plates of the transmission line.

9. An electric transformer as claimed in claim 2, characterised in that said other winding has the plurality of multi-turn coils arranged in equi-angularly spaced apart relationship around said first winding formed from said met-a1 sheet.

10. An electric transformer as claimed in claim 9, characterised in that the plurality of multi-turn coils providing said one other winding are connected electrically in parallel with one another.

11. An electric transformer as claimed in claim 10, characterised by a winding, additional to said one other winding, for providing a measuring potential or a feedback potential.

12. An electric transformer as claimed in claim 1, characterised in that the gore is of rectangular cross-section and is substantially totally enclosed within a rnulti-turn ER A KARL SAALBACH, Primary Examinerwinding of corresponding shape, the multi-turn coils of LVEZEAU, Asst-5mm Emmi-"en said one other winding being located along the straight portions of said core and enclosing Winding. U s CL References Cited 33 -3 8 182 UNITED STATES PATENTS 2,655,623 9/1953 Parker 336-182 X 

